Process for the preparation of a bicyclic decalin ketone

ABSTRACT

Perhydro-5,5-8a-trimethyl-2-naphthalenone is prepared by acid cyclization of carboxylic esters of formula ##STR1## having a double bond in one of the positions indicated by the dotted lines and wherein symbol R designates a C 1  -C 6  alkyl radical, X stands for a monovalent radical of formula P(O)(OR 1 ) 2  or C(O)R 2 , R 1  represents a C 1  -C 6  alkyl group and R 2  is either a linear or branched alkyl group or a substituted or unsubstituted phenyl radical, and wherein the wavy line represents a C--C bond of cis or trans configuration, or of formula ##STR2## wherein the wavy lines and symbol R are defined as above, and R 0  represents a C 3  -C 6  alkyl radical, preferably branched, followed by basic decarboxylation of the obtained product.

BRIEF SUMMARY OF THE INVENTION

The instant invention relates to a process for the preparation ofperhydro-5,5,8a-trimethyl-2-naphthalenone essentially in its isomericform of formula ##STR3## which process comprises treating with an acidiccyclization agent a. a carboxylic ester of formula ##STR4## having adouble bond in one of the positions indicated by the dotted lines andwherein symbol R designates a C_(1-C) ₆ alkyl radical, X stands for amonovalent radical of formula P(O)(OR¹)₂ or C(O)R², R¹ represents aC_(1-C) ₆ alkyl group and R² is either a linear or branched alkyl groupor a substituted or unsubstituted phenyl radical, and wherein the wavyline represents a C--C bond of cis or trans configuration, or

b. a carboxylic ester of formula ##STR5## wherein the wavy lines andsymbol R are defined as above, and R⁰ represents a C_(3-C) ₆ alkylradical, preferably branched, and decarboxylating then the thus obtainedcyclization product by treating it with a base.

This invention provides further carboxylic esters of formula ##STR6## orof formula ##STR7##

More particularly, it provides the following carboxylic esters:

methyl 5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-propionoxypent-2-enoate,

methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(2-methylpropionoxy)-pent-2-enoate

methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(2,2-dimethylpropionoxy)-pent-2-enoate,

methyl 5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-benzoyl-oxypent-2-enoate,and

methyl 7,11-dimethyl-3-(2,2-dimethylpropionoxy)dodeca-2,6,10-trienoate.

BACKGROUND OF THE INVENTION

This invention relates to the field of organic synthesis, moreparticularly to a process for the preparation of a bicyclic decalinketone, viz. perhydro-5,5,8a-trimethyl-2-naphthalenone. This ketone is auseful starting material in the synthesis ofperhydro-5,5,8a-trimethyl-2-naphthalenyl acetate [Polywood ®; trademarkof Firmenich SA, Geneva, Switzerland], a fragrance speciality wellappreciated for its tenacious and elegant woody note. The ketone itselfpossesses a strong woody and ambery odor.

Owing to its structure, it may occur under two isomeric forms of formula##STR8## their reduction followed by acetylation yielding the desiredacetate.

In actual experience it has become apparent that, among the possibleisomers, perhydro-5,5,8aα-trimethyl-2α-trans-naphthalenyl acetate is thecompound which possesses the most interesting olfactive characters,isomer 2β developing instead a note which, though still woody andambery, is less rich than the 2α isomer.

French patent 15 93 814 discloses a method for the preparation ofketones (Ia, b) which method is based on the oxidation of a decalincarbinol obtained as follows: ##STR9##

On account of the above, our efforts were directed to the preparation ofperhydro-5,5,8a-trimethyl-2-naphthalenone in its trans isomeric form.

The present invention provides a solution to this problem.

THE INVENTION

One of the objects of the present invention is a process for thepreparation of perhydro-5,5,8a-trimethyl-2-naphthalenone essentially inisomeric form (Ia), which process comprises treating with an acidiccyclization agent

a. a carboxylic ester of formula ##STR10## having a double bond in oneof the positions indicated by the dotted lines and wherein symbol Rdesignates a C_(1-C) ₆ alkyl radical, X stands for a monovalent radicalof formula P(O)(OR¹)₂ or C(O)R², R¹ represents a C_(1-C) ₆ alkyl groupand R² is either a linear or branched alkyl group or a substituted orunsubstituted phenyl radical, and wherein the wavy line represents aC--C bond of cis or trans configuration, or

b. a carboxylic ester of formula ##STR11## wherein the wavy lines andsymbol R are defined as above, and R⁰ represents a C_(3-C) ₆ alkylradical, preferably branched, and decarboxylating then the thus obtainedcyclization product by treating it with a base.

Suitable acidic cyclization agents include mineral or organic proticacids, or Lewis type acids. Typically, one can e.g. use sulfuric,phosphoric, chlorosulfonic, p-toluenesulfonic or formic acid, whereasamong the Lewis type acids one could mention BF₃. The cyclization canalso be effected by means of ion exchange resins.

In practice, it has become apparent that the cyclization reactionoccurred with the best rate when the proportion of the acid was of about2 equivalents for 1 equivalent of the starting ester. The use of higherproportions however does not exert a marked influence on the observedyields.

With regard to the temperature, the cyclization is carried out at about0°-25° C., preferably at about 5°-10° C.

The subsequent reaction of decarboxylation is effected by analogy withusual techniques by means of known reagents, for instance by means of abase such as an alkali metal hydroxide, e.g. potassium hydroxide,preferably in an alcoholic or an aqueous alcoholic solution, or sodiumhydroxide.

As starting materials of formula (II) and (III), there is used an alkylcarboxylic ester chosen for example among the alkyl esters of thefollowing acids:

5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(diethoxyphosphoroxy)-pent-2-enoic,

5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-propionoxypent-2-enoic,

5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(2-methylpropionoxy)-pent-2-enoic,

5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(2,2-dimethylpropionoxy)-pent-2-enoic,

5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-benzoyloxypent-2-enoic, as wellas their isomers 5-(2,6,6-trimethyl-2-cydohexen-1-ylic)-, and the alkylester of 7,11-dimethyl-3-(2,2-dimethylpropionoxy)-dodeca-2,6,10-trienoicadd.

Preferred esters include the methyl, ethyl and propyl ester derivatives,the former being more preferred. As indicated above, these compounds canoccur in the cis or trans configuration, owing to the relative positionof the carboxylic group with regard to the OX group (compound II) orOC(O)R⁰ (compound III).

We have observed that the best yields are obtained by the cyclization ofthe trans esters and that consequently the process is preferably carriedout with this type of compounds or with mixtures wherein the contents ofthe trans isomer is preponderant. Selectivity is equally influenced bythe configuration of the starting material, the contents of the transisomer of perhydro-5,5,8a-trimethyl-2-naphthalenone (compound Ia) ishigher when the cylization is carried out on the esters having transconfiguration.

Esters (II) are novel compounds and as such they constitute also anobject of the invention.

They can be obtained starting from carboalkoxydihydro-ionones by asimple method by analogy with known technics, as shown by the followingreaction scheme. ##STR12##

By analogy, esters (III) can be obtained starting fromcarboalkoxygeranyl ketone ##STR13##

With the exception of compound (III) wherein R⁰ =R=CH₃ [see Chem. Comm.1992, 503], compound (III) are new compounds and as such they are alsoan object of this invention.

The following examples will show the invention in a more detailedmanner. The temperatures are indicated in degrees centigrade and theabbreviations have the meaning common in the art.

EXAMPLE 1

Carboxylic esters of formula (II) and (III) have been cyclized accordingto the following general procedure.

1 g of starting ester in 1 ml of toluene was added dropwise at 2° to amixture of 2 equivalents of 98% aqueous sulfuric acid in 9 ml oftoluene.

After 2 h, the reaction mixture was poured in a saturated aqueoussolution of sodium bicarbonate and extracted with toluene.

The following step of decarboxylation was carried out on the keto-esterthus obtained by treating it with an alkali metal hydroxide, for examplewith NaOH.

The following table summarizes the results obtained by the conversion ofthe esters indicated.

    ______________________________________                                                decalin ketone                                                        Ia               Ib      Yield [%]                                            ______________________________________                                        (a)    98            2       81                                               (b)    95            5       72                                               (c)    95            5       49                                               (d)    95            5       61                                               (e)    97            3       79                                               (f)    96            4       70                                               ______________________________________                                         (a) = methyl                                                                  (E)5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(diethoxy-phosphoroxy)-pent-2-    noate,                                                                         (b) = methyl                                                                  (Z)5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(diethoxy-phosphoroxy)-pent-2-    noate,                                                                         (c) = methyl                                                                  5(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-propionoxypent-2-enoate,               (d) = methyl                                                                  5(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(2-methylpropionoxy)-pent-2-enoate     (e) = methyl                                                                  5(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(2,2-dimethyl-propionoxy)-pent-2-e    oate                                                                           (f) = methyl                                                                  5(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-benzoyloxypent-2-enoate           

The preparation of the starting esters identified above can be effectedas follows:

Preparation of compound (a)

2.3 g (12.9 mM) of diethylchlorophosphate were added under nitrogen at2° to a stirred solution of 2.5 g (9.4 mM) of methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-oxopentanoate and 1.3 g (12.8mM) of triethylamine in 25 ml of N-methyl-pyrrolidone containing 0.04 g(0.3 mM) of 4-(N,N-dimethylamino)-pyridine.

After 1 h at room temperature, the reaction mixture was poured in asaturated aqueous solution of NH₄ Cl and extracted with ether. Theevaporation of the dried combined organic extracts gave a residue whichby column chromatographic separation (SiO₂ ; cyclohexane/AcOEt 1.5:1)and vacuum distillation yielded a yellowish oil (2.4 g, 65%).

B.p. (bulb-to-bulb distillation oven): 200°-220°/6.6 Pa

Rf (cyclohexane/AcOEt 1.5:1) :0.36.

IR(CHCl₃): 2933, 1717, 1646, 1437, 1371, 1274, 1162, 1126, 1036.

NMR(¹ H): 1.04(s, 6H); 1.39(t, J=7, 6H); 1.43(2H); 1.58(2H); 1.69(s,3H); 1.93(t, J=6, 2H); 2.27(2H); 2.85(2H); 3.71(s, 3H); 4.22(dq, J=7, 7,4H); 5.86(broad s, 1H) δ ppm.

NMR(¹³ C): 166.7(8)(s); 135.8(s); 128.7(s); 104.6(d); 64.8(9)(t);51.3(q); 39.9(t); 35.1(s); 32.9(t); 32.6(t); 28.5(q); 25.8(t); 19.7(q);19.5(t); 16.1(q) δ ppm.

MS: 388(5, M⁺), 252(41), 220(69), 192(29), 155(100), 127(37), 99(65).

Preparation of compound (b)

A solution of 2.5 g (9.4 mM) of methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)3-oxopentanoate in 10 ml oftetrahydrofurane (THF) was added dropwise at room temperature to astirred mixture of NaH (80% in mineral oil; 0.3 g; 0.01M) in 25 ml ofTHF.

After 1 h at room temperature, 2.3 g (12.9 mM) of diethylchlorophosphatewere added dropwise to the mixture.

After 50 min the mixture was cooled and poured carefully into a coldsaturated aqueous solution of NH₄ Cl and extracted with ether.

The usual work up gave a residue which by column chromatography (SiO₂ ;cyclohexane/AcOEt 2:1) followed by distillation yielded 3.1 g (yield84%) of a colorless oil.

B.p. (bulb-to-bulb distillation oven): 200°-220°/6.6 Pa

Rf (cyclohexane/AcOEt 1.5:1):0.29.

IR(CHCl₃): 2934, 1725, 1664, 1436, 1274, 1201, 1149, 1032.

NMR(¹ H): 1.00(s, 6H); 1.37(t, J=7, 6H); 1.41(2H); 1.57(2H); 1.61(s,3H); 1.91(broad t, J=7, 2H); 2.26(2H); 2.48(2H); 3.71(s, 3H); 4.27(dq,J=7, 7, 4H); 5.41(s, 1H) δ ppm.

NMR(¹³ C): 164.4(s); 162.5(s); 162.4(s); 135.5(s); 128.8(s);104.5(6)(t); 64.7(8)(t); 51.1(q); 39.9(t); 35.7(t); 35.0(s); 32.8(t);28.5(2q); 28.1(t); 19.8(q); 19.5(t); 16.1(2q) δ ppm.

MS: 388(1, M⁺), 252(100), 220(68), 192(28), 155(53), 99(48).

Preparation of compound (c)

1.0 g (10.8 mM) of propionyl chloride was added at room temperaturedropwise within 15 min to a stirred solution of 2.5 g (9.4 mM) of methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-oxopentanoate and 1.1 g (10.8mM) of triethylamine in 30 ml of toluene.

After 2 h at room temperature, the mixture was poured into a saturatedaqueous solution of NH₄ Cl and extracted with toluene.

The usual work up, followed by vacuum distillation gave the desiredproduct under the form of an isomeric mixture trans/cis 1:1 (colorlessviscous oil; 1.6 g; yield 55%).

B.p.: 125°-140°/33 Pa

IR(CHCl₃):2940, 1760, 1718, 1436, 1360, 1230, 1138.

The trans isomer showed the following characteristics:

R_(f) (cyclohexane/AcOEt 9:1):0,45.

NMR(¹ H): 1.02(s, 6H); 1.22(t, J=7, 3H); 1.42(2H); 1.56(2H); 1.65(s,3H); 1.91(2H); 2.19(2H); 2.49(q, J=7, 2H); 2.86(2H); 3.72(s, 3H);5.67(s, 1H) δ ppm.

NMR(¹³ C): 171.4(s); 167.1(s); 166.2(s); 136.0(s); 128.6(s); 109.2(d);51.3(q); 39.9(t); 35.1(s); 32.8(t); 32.0(t); 28.5(2q); 27.7(t); 25.7(t);19.7(q); 19.5(t); 8.9(q) δ ppm.

MS: 308(0, M⁺), 219(8), 137(90), 95(48), 81(35), 57(100).

The cis isomer was characterized by the following parameters:

R_(f) (cyclohexane/AcOEt 9:1):0,36.

NMR(¹ H): 0.98(s, 6H); 1.24(t, J=7, 3H); 1.42(2H); 1.56(2H); 1.59(s,3H); 1.91(2H); 2.20(2H); 2.32(2H); 2.57(q, J=7, 2H); 3.68(s, 3H);5.63(s, 1H) δ ppm.

NMR(¹³ C): 171.7(s); 164.5(s); 164.0(s); 135.6(s); 128.5(s); 106.5(d);51.2(q); 39.8(t); 36.1(s); 35.0(s); 32.9(t); 28.5(2q); 27.9(t); 25.2(t);19.6(q); 19.5(t); 8.0(q) δ ppm.

MS:308(0, M⁺), 219(12), 137(49), 95(41), 81(31), 57(100).

Compound (d) was prepared according to the same procedure as thatdescribed hereinabove by using 2-methylpropionyl chloride as reagent.Compound (d) was thus obtained in the form of a trans/cis (3.3:1)isomeric mixture; 2.0 g; yield 66%.

B.p.: 130°-136°/40 Pa

IR(CHCl₃) :2936, 1752, 1718, 1662, 1437, 1361, 1232, 1099.

Isomer trans:

R_(f) (cyclohexane/AcOEt 9:1):0.48.

NMR(¹ H): 1.02(s, 6H); 1.26(d, J=7, 6H); 1.42(2H); 1.56(2H); 1.65(s,3H); 1.91(2H); 2.19(2H); 2.69(m, 1H); 2.86(2H); 3.72(s, 3H); 5.65(s, 1H)δ ppm.

NMR(¹³ C): 174.3(s); 167.2(s); 166.2(s); 135.9(s); 128.6(s); 109.1(d);51.3(q); 40.0(t); 35.1(s); 34.4(d); 32.9(t); 31.9(t); 28.5(2q); 25.7(t);19.7(q); 19.5(t); 16.9(2q) δ ppm.

MS: 322(4, M⁺), 234(9), 219(7), 137(81), 71(100).

Isomer cis:

R_(f) (cyclohexane/AcOEt 9:1):0.39.

NMR(¹ H): 0.98(s, 6H); 1.29(d, J=7, 6H); 1.42(2H); 1.56(2H); 1.58(s,3H); 1.91(2H); 2.19(2H); 2.29(2H); 2.78(m, 1H); 3.67(s, 3H); 5.62(s, 1H)δ ppm.

NMR(¹³ C): 174.3(s); 167.2(s); 164.5(s); 135.6(s); 128.6(s); 106.8(d);51.1(q); 39.8(t); 36.0(t); 35.0(s); 34.2(d); 32.8(t); 28.5(2q); 25.3(t);19.7(q); 19.5(t); 16.9(2q) δ ppm.

MS: 322(0, M⁺), 234(7), 219(11), 137(42), 71(100).

Compound (e) was prepared according to the same procedure as thatdescribed hereinabove by using 2,2-dimethylpropionyl chloride asreagent. Compound (e) was obtained in the form of a trans/cis (>9:1)isomeric mixture; 2.6 g; yield 82%.

B.p.: 140°-145°/40 Pa

IR(CHCl₃):2934, 1745, 1717, 1660, 1436, 1361, 1216, 1098.

Isomer trans:

R_(f) (cyclohexane/AcOEt 9:1):0.52.

NMR(¹ H): 1.02(s, 6H); 1.30(s, 9H); 1.42(2H); 1.57(2H); 1.66(s, 3H);1.92(2 H); 2.20(2H); 2.87(2H); 3.72(s, 3H); 5.62(s, 1H) δ ppm.

NMR(¹³ C): 175.8(s); 167.5(s); 166.2(s); 135.9(s); 128.6(s); 109.0(d);51.3(q); 40.0(t); 39.3(s); 35.1(s); 32.9(t); 31.9(t); 28.5(2q);27.1(3q); 25.7(t); 19.7(q); 19.5(t) δ ppm.

MS: 336(0, M⁺), 137(31), 95(12), 85(19), 57(100).

Isomer cis:

R_(f) (cyclohexane/AcOEt 9:1):0.44.

NMR(¹ H): 0.98(s, 6H); 3.67(s, 3H) δ ppm.

Preparation of compound (f)

1.5 ml (12.9 mM) of benzoyl chloride were added dropwise at 2° in anitrogen atmosphere to a stirred solution of 2.5 g (9.4 mM) of methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-oxopentanoate, 1.3 g (12.8 mM)of triethylamine in 30 ml of toluene containing 0.04 g (0.3 mM) of4-(N,N-dimethylamino)pyridine.

After 2.5 h at room temperature, the reaction mixture was poured into asaturated aqueous solution of NH₄ Cl and extracted with ether.

The usual work up followed by distillation gave desired ester (f) in theform of a trans/cis (53:47) isomeric mixture: 3.3 g; yield 93%.

A chromatographic separation on column allowed to obtain compound (f) inthe pure trans and cis isomeric form.

Isomer trans:

B.p. (bulb-to-bulb distillation oven): 200°-220°/8 Pa

R_(f) (cyclohexane/AcOEt 9:1):0.43.

IR(CHCl₃):2932, 1735, 1661, 1437, 1361, 1261, 1203, 1159, 1104, 1067,1028.

NMR(¹ H): 1.00(s, 6H); 1.40(2H); 1.55(2H); 1.62(s, 3H); 1.89(broad t,J=7, 2H); 2.29(m, 2H); 3.01(m, 2H); 3.75(s, 3H); 5.83(s, 1H); 7.50(t,J=8, 2H); 7.63(broad t, J=7, 1H); 8.10(broad d, J=7.5, 2H) δ ppm.

NMR(¹³ C): 167.2(s); 166.2(s); 164.0(s); 135.9(s); 133.8(d); 130.1(2d);129.2(s); 128.7(2d); 128.6(s); 109.5(d); 51.4(q); 39.9(t); 35.0(s);32.9(t); 32.0(t); 28.5(2q); 25.7(t); 19.6(q); 19.5(t) δ ppm.

MS: 356(0, M⁺), 137(22), 105(100), 77(16).

Isomer cis:

B.p. (bulb-to-bulb distillation oven): 200°-220°/8 Pa

R_(f) (cyclohexane/AcOEt 9:1):0.34.

IR(CHCl₃): 2933, 1729, 1667, 1437, 1270, 1199, 1175, 1139, 1083, 1066,1026.

NMR(¹ H): 0.99(s, 6H); 1.41(2H); 1.56(2H); 1.64(s, 3H); 1.90(broad t,J=7, 2 H); 2.30(m, 2H); 2.47(m, 2H); 3.61(s, 3H); 5.74(s, 1H); 7.48(t,J=8, 2H); 7.61(t, J=7, 1H); 8.14(broad d, J=7.5, 2H) δ ppm.

NMR(¹³ C): 164.4(s); 163.9(s); 163.8(s); 135.5(s); 133.5(d); 130.2(2d);129.5(s); 128.7(s); 128.6(2d); 107.0(d); 51.3(q); 39.8(t); 36.1(t);35.0(s); 32.8(t); 28.5(2q); 25.2(t); 19.8(q); 19.4(t) δ ppm.

MS: 356(0, M⁺), 241(3), 137(9), 105(100), 77(19).

Methyl 5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-oxopentanoate, used asstarting material in the process above can be obtained according to themethod described by G. Buchi and H. Wuest, Helv. Chim. Acta 72, 996(1989).

EXAMPLE 2

1060 g of 98% aqueous sulfuric acid and 2 l of toluene were poured intoa three necked reaction vessel equipped with a mechanical stirrer, athermometer, a condenser and a dropping funnel. The mixture was cooledto 3°-4°, thereupon 1060 g (3.14M) of methyl7,11-dimethyl-3-(2,2,-dimethylpropionoxy)-dodeca-2,6,10-trienoate whereadded dropwise thereto. During the addition the temperature wascarefully kept below 15°, then the mixture was kept under stirring for1.5 h at 5°-10°. 1 kg of crushed ice was then slowly added to thereaction mixture while the temperature was maintained below 20°.

After decantation, the mixture was washed with a saturated aqueoussolution of sodium bicarbonate and extracted with toluene.

The following step of decarboxylation was carried out by treatment witha sodium hydroxide aqueous solution.

For a quantity of 1060 g (3.13M) ofperhydro-5,5,8a-trimethyl-3-carbomethoxy-2-naphthalenone there were used2720 g of a 30% aqueous solution of NaOH.

423.5 g (1.66M) of perhydro-5,5,8a-trimethyl-2-naphthalenone were thusobtained, the trans isomeric contents of which was 76%.

Other acidic cyclization reactions of methyl7,11-dimethyl-3-(2,2-dimethylpropionoxy)-dodeca-2,6,10-trienoate werecarried out by replacing sulfuric acid by formic acid, phosphoric acidand BF₃ under the form of trifluoroboroetherate.

The starting methyl ester was prepared as described hereinafter startingfrom a trans enriched fraction of geranylacetone.

a. 24 g (0.55M) of sodium hydride (55% dispersion in mineral oil) wereadded at 20° under nitrogen to a solution of 135 g (1.5M) ofdimethylcarbonate in 500 ml of a mixture of toluene/N-methylpyrrolidone(9:1).

The obtained mixture was heated to reflux (bath temperature 100°) then,during 1 h, there was added a solution of 0.5M of geranylacetone in 135g (1.5M) of dimethylcarbonate, where upon the resulting reaction mixturewas kept refluxing during 10 min. It was finally poured into a 10%aqueous solution of NH₄ Cl saturated with NaCl. An ether extraction,followed by the usual work up of the combined organic extracts andfractional distillation gave methyl3-oxo-7,11-dimethyl-dodeca-6,10-dienoate.

b. A mixture of 796 g of the obtained keto-ester in 200 ml of 80-100petrol ether (3.16M) and 349.5 g (3.46M) of triethylamine was charged ina three necked vessel equipped with a thermometer, a dropping funnel anda mechanical stirrer. To the resulting mixture heated to 70°, there wereadded within 2 h under stirring 388 g (3.22M) of pivaloyl chloride.

After cooling, the mixture was washed with two fractions of 900 ml eachof water and the organic phase was concentrated to give 1060 g of methyl7,11-dimethyl-3-(2,2-dimethyl-propionoxy)-dodeca-2,6,10-trienoate, theanalytical characters of which were the following:

NMR(¹ H, 360 MHz, CDCl₃): 1.27 and 1.28(2s, 9H); 1.60 and 1.68(2 broads, 9H); 1.94-2.08(m, 4H); 2.21(q, J=7, 2H); 2.82 and 2.84(2 tr, J=7,2H); 3.71(s, 3H); 5.08 and 5.15(2 broad t, J=7, 2H); 5.65(s, 1H) δ ppm.

NMR(¹³ C, 90.5 MHz, CDCl₃): 15.9(q); 17.7(2q); 23.4(q); 25.3(t);25.7(2q); 26.6(t); 26.7(t); 27.0(6q); 31.2(t); 31.5(t); 31.9(t);39.3(2s); 39.7(2t); 51.3(2q); 109.5(2d); 122.5(2d); 123.4(d); 124.3(d);131.4(s); 131.6(s); 136.6(2s); 166.2(s); 166.3(s); 167.4(2s); 175.8(2s)δ ppm.

MS: 336(0, M³⁰ ), 57(100), 69(36), 85(17), 41(17), 81(10), 109(7),123(6), 67(6), 151(5), 101(5), 136(4), 95(4).

What we claim is:
 1. A process for the preparation ofperhydro-5,5,8a-trimethyl-2-napthalenone essentially in its isomericform of formula ##STR14## which process comprises treating with anacidic cyclization agent, in an inert organic solvent, at a temperatureof about 0° to 25° C.;a. a carboxylic ester of formula ##STR15## havinga double bond in one of the positions indicated by the dotted lines andwherein the symbol R designates a C₁ -C₆ alkyl radical, X is an oxygenprotecting group comprising a monovalent radical of formula P(O)(OR¹)₂or C(O)R², R¹ represents a C₁ -C₆ alkyl group and R² is either a linearor branched alkyl group or a substituted or unsubstituted phenylradical, and wherein the wavy line represents a C--C bond of cis ortrans configuration, or b. a carboxylic ester of formula ##STR16##wherein the wavy lines and symbol R are defined as above, and R⁶represents a C₃ -C₆ alkyl radical, preferably branched, anddecarboxylating then the thus obtained cyclization product by treatingit with a base;wherein the proportion of the cyclization agent relativeto the ester is at least stoichiometric.
 2. A process according to claim1 wherein the cyclization agent is a protic mineral or organic acid or aLewis type add.
 3. A process according to claim 2 wherein the proticacid is sulfuric, phosphoric, chlorosulfonic, p-toluenesulfonic orformic acid and the Lewis type acid is BF₃.
 4. A process according toclaim 1 wherein the carboxylic esters of formula (II) or (III) arepreferably in their isomeric form trans, or in the form of a mixturecontaining essentially said trans isomer together with minor amounts ofits cis isomer.
 5. A process according to claim 4 wherein the carboxylicester is a member of the group constituted by:methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(diethoxyphosphoroxy)-pent-2-enoate,methyl 5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-propionoxypent-2-enoate,methyl5-(2,6,6-trimethyl-1-cydohexen-1-yl)-3-(2-methylpropionoxy)-pent-2-enoate,methyl5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(2,2-dimethylpropionoxy)-pent-2-enoate,methyl 5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-benzoyl-oxypent-2-enoate,and methyl7,11-dimethyl-3-(2,2-dimethylpropionoxy)dodeca-2,6,10-trienoate.
 6. Aprocess according to claim 5 wherein the carboxylic esters areessentially in their trans isomeric configuration.
 7. A processaccording to claim 1 wherein the acid cyclization agent is used in theproportion of 2 equivalents for 1 equivalent of carboxylic esterstarting material.
 8. A process according to claim 1 wherein thecyclization is effected at a temperature of between about 0° and 25° C.9. A process according to claim 1 wherein the decarboxylation iseffected by treating the obtained cyclization product with an alkalimetal hydroxide, preferably KOH in an aqueous-alcoholic solution orNaOH.